This invention generally relates to head-mounted display systems for viewing electronically generated stereoscopic images and more particularly relates to an apparatus and method for an arrangement of optical components to provide a head-mounted optical apparatus that provides a very wide field of view and large exit pupils.
The potential value of stereoscopic display systems is widely appreciated particularly in entertainment and simulation fields. An overall goal is to provide the most realistic display possible, within the constraints of a particular application. Realistic stereoscopic display is characterized by a number of quantifiable features, including wide field of view (horizontal and vertical), sharp image resolution, and sufficient brightness.
One alternative for stereoscopic imaging is provided by head-mounted displays (HMDs). Goggles, glasses, and similar eyewear have been used in a number of ways to provide compact, lightweight, and inexpensive head-mounted display apparatus for providing stereoscopic effects. Solutions using eyewear range from using lenses having different polarization filters for each eye, as is disclosed, for example, in U.S. Pat. No. 6,034,717 (Dentinger et al.), to using shutters that alternate between right and left eye visibility as is disclosed, for example, in U.S. Pat. No. 4,021,846 (Roese). An alternate shutter mechanism is a color shutter, as disclosed in U.S. Pat. No. 5,903,395 (Rallison et al.) which provides varying color filters at high frequencies. Solutions of this type can be economical, particularly when used to accommodate a large group of observers. However, the requirement for using a separate external display, such as a display screen or CRT, means that such solutions necessarily have a large footprint and are, therefore, not well suited for individual viewing. Thus, it can be seen that there are advantages to an eyewear arrangement in which the displayed stereoscopic image is contained within the head-mounted display itself.
There are numerous HMDs and face-mounted devices (FMDs) commercially available, offering either stereoscopic or non-stereoscopic imaging. One example of an HMD that provides stereoscopic imaging is the Cyber Eye CE 200S from iReality.com, Inc., Half Moon Bay, Calif. Another example is the Datavisor 80(trademark) Wide Field-of-View Head Mounted Display from n-vision, Inc., McLean, Va. An HMD providing non-stereoscopic imaging is the Eye-Treck(trademark), a product of Olympus Optical Company, Ltd., Tokyo, Japan.
While HMD devices that are currently available provide suitable imaging for some applications, there is room for improvement. For example, realistic stereoscopic imaging requires a wide field of view. However, existing devices are limited in this regard. The Cyber Eye CD 200S HMD, for example, claims a field of view limited to 22.5 degrees horizontal, 16.8 degrees vertical. The Eye-Treck HMD claims a field of view of less than 40 degrees horizontal, less than 22 degrees vertical. The Datavisor 80 device, while it claims a field of view of 120 degrees horizontal, is relatively large and is too expensive for most applications. The resolution of commercially available devices is also limited. Therefore, while image quality may be acceptable for some applications such as for computer monitor display, the performance level achieved by existing devices is not sufficient for a broader range of virtual reality applications.
Product literature from one HMD manufacturer (iReality.com, Inc. at www.ireality.com/hmds.html) states that sharp resolution and wide field-of-view imaging can be considered xe2x80x9cmutually exclusivexe2x80x9d for HMD design. Certainly, the difficulty of providing sharp resolution, wide field-of-view imaging in a compact, lightweight HMD is well-appreciated in the optical design arts and conventional solutions have made only modest improvements in obtaining realistic stereoscopic imaging. In the optical design arts, it is well understood that HMD design presents a number of strict constraints, with a number of optical parameters that are fixed and inflexible. Using a conventional approach to this type of optical design problem, the configuration of the Eye-Treck HMD employs a free-shaped, aspheric prism in order to provide an optics path having compact dimensions with minimal aberrations and image distortion. Aspheric design approaches are known to have particular value because they can allow an optical designer considerably more variables to work with than optics using more conventional spherical structures. However, aspheric design can present significant drawbacks relative to cost, design complexity, and overall flexibility of a design solution. For example, modification or scaling of an aspheric design solution may not be possible without considerable redesign. Examples of HMD devices based on aspheric optical components are disclosed in U.S. Pat. No. 6,028,709 (Takahashi) which discloses an HMD using a prism having rotationally asymmetric surfaces; U.S. Pat. No. 6,097,354 (Takahashi et al.); U.S. Pat. No. 5,436,765 (Togino) which discloses use of an aspherical mirror; U.S. Pat. No. 5,959,780 (Togino et al.); and U.S. Pat. No. 5,596,433 (Konuma) which discloses use of a beamsplitter having an aspherical surface as HMD component.
Spherical optics, on the other hand, present other problems to the HMD designer. Most notably, the image to be presented to an observer is conventionally provided on a flat surface. Spherical optics can require considerable design in order to compensate for image aberration and distortion that results from attempting to project a flat image using substantially spherical lenses. This difficulty is seen, for example, in the design disclosed in U.S. Pat. No. 5,903,395 (Rallison et al.) noted above, which discloses conventional lens design approaches to project an image from a flat surface. At the same time, however, it can be appreciated that there are advantages to the use of spherical optics, particularly with respect to availability, manufacturability, and cost. Additional potential advantages over aspherical approaches include scalability and flexibility of design when using spherical optics.
One example of an HMD design utilizing spherical optics is the pilot""s helmet disclosed in U.S. Pat. No. 4,854,688 (Hayford et al.), directed to the transmission of a non-stereoscopic, two-dimensional image along a non-linear path, such as attached to headgear for a pilot. In the optical arrangement of the Hayford et al. patent, a ball lens, employed as a folding component and lens for providing a relatively wide field of view, directs a collimated output image, optically at infinity, for viewing by a pilot. This allows a pilot to view information projected from a small CRT screen without requiring change of gaze direction or refocusing. While the ball lens may provide some advantages for inherently wide field of view in the device disclosed in U.S. Pat. No. 4,854,688, there are a number of drawbacks to a design of this type. The tilted and decentered optics path of the disclosed device in U.S. Pat. No. 4,854,688 inherently introduces image aberrations that require a considerable amount of correction. To compensate for off-axis imaging from a reflective or partially reflective collimator, a complex and costly relay lens assembly, tilted and decentered with respect to the optical path, are required. Relay optics of the type disclosed in the Hayford et al. patent, needed for such a compact arrangement, not only add cost, but also increase the bulk and weight of a head-mounted viewing device.
Imaging by forming a large pupil has particular advantages when providing an HMD solution with a wide field of view. However, with conventional lens design approaches, forming a large pupil inherently causes other problems, since the lens design must attempt to correct for the pupil as well as for the wide field. The lens system must not only be able to correct for on-axis aberrations (chiefly spherical aberration and axial chromatic aberration), but for off-axis aberrations as well, including coma, astigmatism, field curvature, and distortion, and chromatic lateral aberrations. Therefore, conventional lens design approaches do not yield simple solutions for correcting aberrations when providing a large pupil with a wide field of view.
Another well known problem in achieving wide field of view using conventional optics systems is illumination falloff due to the Cosine Fourth Law, in which image brightness is reduced at a rate proportional to cos4 of the off-axis field angle. This effect can detract from realistic appearance desired for an immersive experience.
Monocentric optical design has advantages for providing, using a straightforward optical arrangement, design solutions that perform well and inherently require a minimum of components. Ideally, a monocentric arrangement offers optimal performance for field of view with minimal distortion and image aberration. However, monocentric design of the optical path is difficult given the constraints of HMD design and has not, therefore, been employed successfully in conventional HMD design practices. Instead, existing HMD solutions are characterized by complex optical paths such as that presented in U.S. Pat. No. 4,854,688, limited field of view, high cost, and overall low image resolution.
Some HMDs display a real image, projected onto a screen surface, within the HMD itself, such as the goggles disclosed in U.S. Pat. No. 5,506,705 (Yamamoto et al.) However, designs of this type typically have inherent disadvantages relating to potential eyestrain and proper correlation of visual cues, such as vergence and accommodation, particularly since any type of display surface must be in close proximity to the eye of the observer. It is recognized that there can be advantages in an alternate approach in which, instead of a real image, a virtual image is presented to the observer. Visual cues for virtual imaging allow the observer to focus more naturally and provide inherently more realistic viewing conditions.
Thus it can be seen that there is a need for an improved, compact head-mounted display that provides a virtual image at a large pupil, with a wide field of view and sharp resolution, using a substantially monocentric optical design.
It is an object of the present invention to provide a compact, lightweight, and affordable head-mounted display that provides a wide field of view and sharp resolution. With this object in mind, the present invention provides a head-mounted display apparatus for viewing a composite virtual image, said composite virtual image comprising a left virtual image, formed from a left image, to be viewed by an observer at a left viewing pupil and a right virtual image, formed from a right image, to be viewed by the observer at a right viewing pupil, the head-mounted display apparatus comprising:
(a) an image generator for providing, as scene content data, the left image and the right image;
(b) a left image display for accepting from the image generator the scene content data for the left image and for displaying a first left intermediate image on a left curved surface;
(c) a left ball lens spaced apart from the left curved surface, the center of curvature of the left ball lens substantially coincident with a center of curvature of the left curved surface, the left ball lens having a left ball lens pupil, the left ball lens forming a second left intermediate image of the first left intermediate image;
(d) a right image display for accepting from the image generator the scene content data for the right image and for displaying a right intermediate image on a right curved surface;
(e) a right ball lens spaced apart from the right curved surface, the center of curvature of the right ball lens substantially coincident with a center of curvature of the right curved surface, the right ball lens having a right ball lens pupil, the right ball lens forming a second right intermediate image of the first right intermediate image;
(f) a beamsplitter means disposed to do the following:
(1) reflect the second left intermediate image formed by the left ball lens towards a left curved mirror, the left curved mirror having its center of curvature optically coincident with the left ball lens, the left curved mirror disposed to form, through the beamsplitter, a real image of the left ball lens pupil at the left viewing pupil and a left virtual image of the second left intermediate image;
(2) reflect the second right intermediate image directed from the right curved surface through the right ball lens towards a right curved mirror, the right curved mirror having its center of curvature optically coincident with the right ball lens, the right curved mirror disposed to form, through the beamsplitter, a real image of the right ball lens pupil at the right viewing pupil and a right virtual image of the second right intermediate image.
A feature of the present invention is the use of a ball lens assembly for projecting the image viewed by each eye.
A further feature of the present invention is the generation of an intermediate image having a spherical curvature that suits the projection of the intermediate image by a ball lens onto a mirror having spherical curvature.
An alternative feature of the present invention is the use of an optical component to provide spherical curvature to an intermediate image for projection by a ball lens.
It is an advantage of the present invention that it provides a virtual image to the observer. The observer enjoys a wide field of view, with minimal distortion and minimal aberrations in the projected image.
It is a further advantage of the present invention that it provides pupil imaging with a large pupil size for HMD applications.
It is a further advantage of the present invention that it employs a monocentric optical arrangement, which inherently minimizes image aberrations and distortion. The design of the preferred embodiment even allows an essentially concentric arrangement of optical components, with a separate optics path for each eye of the observer.
It is a further advantage of the present invention that it provides a wider field of view than is conventionally available at reasonable cost for HMD devices. Moreover, the arrangement of the present invention, because it employs ball lens optics, does not suffer the brightness degradation, due to the Cosine Fourth Law, common to conventional wide-field imaging optics.
These and other objects, features, and advantages of the present invention will become apparent to those skilled in the art upon a reading of the following detailed description when taken in conjunction with the drawings wherein there is shown and described an illustrative embodiment of the invention.